It's the tuned muffler that causes this situation. Put a OS muffler on the engine and the needle valve will work like one on a OS engine. The Tower muffler doesn't make much muffler pressure.

Also, if your low speed needle is screwed in too far, it will desensitize the high speed needle on most brands.

The Tower with the tuned muffler tends to have a rich mid range. Someone could have been attempting to make the mid range correct? Therefore the wide open throttle would be lean.

Enjoy,

Jim